Structural disorder and its effect on the superconducting transition temperature in the organic superconductor k-„BEDT-TTF...2Cu†N„CN...2‡Br

In this paper, we report direct evidence of a structural transition in the organic superconductor k-~BEDT-TTF!2Cu@N~CN!2#Br near 80 K and the effect of disorder on the superconducting transition temperature. By cooling the sample from above 80 K, the interlayer magnetoresistance displays a bumplike feature, which increases sharply with increasing cooling rate. The rapidly cooled sample has a much larger resistivity and a lower transition temperature, which decreases linearly with increasing resistivity near the transition temperature. We propose that rapid cooling quenches the sample into a disordered state. Localized moments in the disordered state reduce the superconducting transition temperature. @S0163-1829~98!51522-6#